implement derive_into for scrypt

CHANGELOG.rst

@@ -48,8 +48,9 @@ Changelog
 * Added ``derive_into`` methods to
   :class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDF`,
   :class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDFExpand`,
-  :class:`~cryptography.hazmat.primitives.kdf.argon2.Argon2id`, and
-  :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC` to allow
+  :class:`~cryptography.hazmat.primitives.kdf.argon2.Argon2id`,
+  :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC`, and
+  :class:`~cryptography.hazmat.primitives.kdf.scrypt.Scrypt` to allow
   deriving keys directly into pre-allocated buffers.
 * Added ``encrypt_into`` methods to
   :class:`~cryptography.hazmat.primitives.ciphers.aead.AESCCM`,

docs/hazmat/primitives/key-derivation-functions.rst

@@ -418,13 +418,40 @@ Scrypt
         :raises TypeError: This exception is raised if ``key_material`` is not
                            ``bytes``.
         :raises cryptography.exceptions.AlreadyFinalized: This is raised when
-                                                          :meth:`derive` or
+                                                          :meth:`derive`,
+                                                          :meth:`derive_into`, or
                                                           :meth:`verify` is
                                                           called more than
                                                           once.
 
         This generates and returns a new key from the supplied password.
 
+    .. method:: derive_into(key_material, buffer)
+
+        .. versionadded:: 47.0.0
+
+        :param key_material: The input key material.
+        :type key_material: :term:`bytes-like`
+        :param buffer: A writable buffer to write the derived key into. The
+                       buffer must be equal to the length supplied in the
+                       constructor.
+        :type buffer: :term:`bytes-like`
+        :return int: the number of bytes written to the buffer.
+        :raises ValueError: This exception is raised if the buffer length does
+                           not match the specified ``length``.
+        :raises TypeError: This exception is raised if ``key_material`` or
+                           ``buffer`` is not ``bytes``.
+        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
+                                                          :meth:`derive`,
+                                                          :meth:`derive_into`, or
+                                                          :meth:`verify` is
+                                                          called more than
+                                                          once.
+
+        This generates a new key from the supplied password and writes it into
+        the provided buffer. This is useful when you want to avoid allocating
+        new memory for the derived key.
+
     .. method:: verify(key_material, expected_key)
 
         :param bytes key_material: The input key material. This is the same as
@@ -436,7 +463,8 @@ Scrypt
                                                     derived key does not match
                                                     the expected key.
         :raises cryptography.exceptions.AlreadyFinalized: This is raised when
-                                                          :meth:`derive` or
+                                                          :meth:`derive`,
+                                                          :meth:`derive_into`, or
                                                           :meth:`verify` is
                                                           called more than
                                                           once.

src/cryptography/hazmat/bindings/_rust/openssl/kdf.pyi

@@ -31,6 +31,7 @@ class Scrypt:
         backend: typing.Any = None,
     ) -> None: ...
     def derive(self, key_material: Buffer) -> bytes: ...
+    def derive_into(self, key_material: Buffer, buffer: Buffer) -> int: ...
     def verify(self, key_material: bytes, expected_key: bytes) -> None: ...
 
 class Argon2id:

src/rust/src/backend/kdf.rs

@@ -144,6 +144,50 @@ struct Scrypt {
     used: bool,
 }
 
+impl Scrypt {
+    #[cfg(not(CRYPTOGRAPHY_IS_LIBRESSL))]
+    fn derive_into_buffer(
+        &mut self,
+        py: pyo3::Python<'_>,
+        key_material: &[u8],
+        output: &mut [u8],
+    ) -> CryptographyResult<usize> {
+        if self.used {
+            return Err(exceptions::already_finalized_error());
+        }
+        self.used = true;
+
+        if output.len() != self.length {
+            return Err(CryptographyError::from(
+                pyo3::exceptions::PyValueError::new_err(format!(
+                    "buffer must be {} bytes",
+                    self.length
+                )),
+            ));
+        }
+
+        openssl::pkcs5::scrypt(
+            key_material,
+            self.salt.as_bytes(py),
+            self.n,
+            self.r,
+            self.p,
+            (usize::MAX / 2).try_into().unwrap(),
+            output,
+        )
+        .map_err(|_| {
+            // memory required formula explained here:
+            // https://blog.filippo.io/the-scrypt-parameters/
+            let min_memory = 128 * self.n * self.r / (1024 * 1024);
+            CryptographyError::from(pyo3::exceptions::PyMemoryError::new_err(format!(
+                "Not enough memory to derive key. These parameters require {min_memory}MB of memory."
+            )))
+        })?;
+
+        Ok(self.length)
+    }
+}
+
 #[pyo3::pymethods]
 impl Scrypt {
     #[new]
@@ -214,26 +258,25 @@ impl Scrypt {
         }
     }
 
+    #[cfg(not(CRYPTOGRAPHY_IS_LIBRESSL))]
+    fn derive_into(
+        &mut self,
+        py: pyo3::Python<'_>,
+        key_material: CffiBuf<'_>,
+        mut buf: CffiMutBuf<'_>,
+    ) -> CryptographyResult<usize> {
+        self.derive_into_buffer(py, key_material.as_bytes(), buf.as_mut_bytes())
+    }
+
     #[cfg(not(CRYPTOGRAPHY_IS_LIBRESSL))]
     fn derive<'p>(
         &mut self,
         py: pyo3::Python<'p>,
         key_material: CffiBuf<'_>,
     ) -> CryptographyResult<pyo3::Bound<'p, pyo3::types::PyBytes>> {
-        if self.used {
-            return Err(exceptions::already_finalized_error());
-        }
-        self.used = true;
-
-        Ok(pyo3::types::PyBytes::new_with(py, self.length, |b| {
-            openssl::pkcs5::scrypt(key_material.as_bytes(), self.salt.as_bytes(py), self.n, self.r, self.p, (usize::MAX / 2).try_into().unwrap(), b).map_err(|_| {
-                // memory required formula explained here:
-                // https://blog.filippo.io/the-scrypt-parameters/
-                let min_memory = 128 * self.n * self.r / (1024 * 1024);
-                pyo3::exceptions::PyMemoryError::new_err(format!(
-                    "Not enough memory to derive key. These parameters require {min_memory}MB of memory."
-                ))
-            })
+        Ok(pyo3::types::PyBytes::new_with(py, self.length, |output| {
+            self.derive_into_buffer(py, key_material.as_bytes(), output)?;
+            Ok(())
         })?)
     }
 

tests/hazmat/primitives/test_scrypt.py

@@ -233,3 +233,29 @@ def test_invalid_r(self, backend):
     def test_invalid_p(self, backend):
         with pytest.raises(ValueError):
             Scrypt(b"NaCl", 64, 2, 8, 0, backend)
+
+    def test_derive_into(self, backend):
+        scrypt = Scrypt(b"NaCl", 64, 1024, 8, 16, backend)
+        buf = bytearray(64)
+        n = scrypt.derive_into(b"password", buf)
+        assert n == 64
+        # Verify the output matches what derive would produce
+        scrypt2 = Scrypt(b"NaCl", 64, 1024, 8, 16, backend)
+        expected = scrypt2.derive(b"password")
+        assert buf == expected
+
+    @pytest.mark.parametrize(
+        ("buflen", "outlen"), [(63, 64), (65, 64), (32, 64), (128, 64)]
+    )
+    def test_derive_into_buffer_incorrect_size(self, buflen, outlen, backend):
+        scrypt = Scrypt(b"NaCl", outlen, 1024, 8, 16, backend)
+        buf = bytearray(buflen)
+        with pytest.raises(ValueError, match="buffer must be"):
+            scrypt.derive_into(b"password", buf)
+
+    def test_derive_into_already_finalized(self, backend):
+        scrypt = Scrypt(b"NaCl", 64, 1024, 8, 16, backend)
+        buf = bytearray(64)
+        scrypt.derive_into(b"password", buf)
+        with pytest.raises(AlreadyFinalized):
+            scrypt.derive_into(b"password", buf)